CHERNOBYL, Ukraine — In April 1986, Alexander Rodnyansky was a young documentary filmmaker living in Kyiv. When the fourth reactor of the Chornobyl Nuclear Power Station exploded 60 miles north of the Ukrainian capital, most citizens of the Soviet Union were not informed. It took the government 18 days to share exactly what had happened, but Rodnyansky had been filming the disaster zone from the day after the catastrophe.